1. Don't shoot wildly. Though an unmoded AR has roughly 120 bullets, play like you're trying to conserve your ammo. That will decrease the times you need to reload in combat.
2. If possible, always aim for the chest. The natural recoil of the weapon will usually lead to a headshot and thus you save bullets and health.
3. If you keep running at people head on, you'll die a lot. Try to work the enemy's flanks and if possible, make sure you have a clear shot before taking it. Otherwise you could miss and they'll start shooting you. surprise is key (though I know its hard on small maps like junkflea)
4. When you reload always be prepared to swap to your secondary weapon or try to reload before your clip is completely empty. The reason for the second part is because if you're in the middle of a reload and someone comes around the corner, tapping "1" twice will cancel the reload and swing your weapon back out in which case you still have a few bullets to fight with.
5. It's a lot easier to stay alive if you have coordination with your team, so if you haven't already, think about joining a clan. Voice communications is a great tool.
6. Bunny hopping (as mentioned in an above post) is very effective, but the bunny hopper needs to know how to shoot with the decreased accuracy. Best gun to bunny hop with is the shot gun since with the spray, it hardly matters where you shoot. For reloading purposes, unless you're under fire I don't suggest doing that but if you're in combat, try to bunny hop around walls and poles.
7. Strafing is an excellent way to stay alive. Especially if you're sniping. On junk flea, I can usually go with a 4:1 ratio by sniping on the roof tops. (The single roof top is my preferred spot [ can't remember what side it's on..its the green one]). Most shooters aim at you and not where you're going. So if you keep moving, the chance of them missing is increased.